1. Field of the Invention
The present invention relates to an optical equipment such as a video camera, a still camera and a lens barrel having a lens control unit for controlling a lens position.
2. Related Background Art
Various types of lenses have been used in a zoom lens used for a video camera. FIG. 8 shows a common zoom lens. Numeral 1 denotes a front lens group which is movable along an optical axis for focusing, numeral 2 denotes a variator lens group which varies magnification, and numeral 3 denotes a compensator lens group for maintaining a focus plane constant when the magnification is varied. The variator 2 and the compensator 3 are moved for varying the magnification while they are kept in a predetermined relationship. Numeral 4 denotes a focusing lens group, and numeral 5 denotes a focus plane on which a CCD is mounted. In a front focus lens shown in FIG. 8 in which the focusing is done by the front lens 1, a mechanical cam ring is usually used to keep a fixed relationship between the variator 2 and the compensator 3.
On the other hand, an inner focus or rear focus lens in which the focusing is done by a lens group behind the variator lens rather than by the front focus lens is also known.
In a rear focus zoom lens shown in FIG. 9, the front lens group is fixed while the variator 2 and the compensator 3 are moved in linkage by a mechanical cam ring as they are in the front focus lens of FIG. 8. A lens group 4 or a portion thereof functions as a focusing lens. Where the focusing lens group is located behind the variator lens, it is necessary to move the lens group 4 during zooming even if a distance to an object remains unchanged. Further, a position of the lens group 4 to comply with a focal length varies with the distance to the object. FIG. 10 shows an in-focus position in accordance with a distance to an object for various focal lengths of the lens group 4. An abscissa represents the focal length, W represents a wide end and T represents a telescope end. An ordinate denotes a position of the lens group 4 and curves 6-10 shown in-focus positions for a given distance. For example, curve 6 represents 50 cm, curve 7 represents 1 meter, curve 8 represents 2 meters, curve 9 represents 10 meters and curve 10 represents infinite.
Where the lens group 4 is driven out (toward the object) to the wide end at a point 11, the focusing to a very near object is attained.
Another construction of inner focus lens is shown in FIG. 11. The lens group 1, the variator 2 and the iris 12 are moved during the magnification variation. Numeral 4A denotes a fixed lens group and numeral 4B denotes a focusing lens group.
Since there is no compensation lens group (the compensator in FIG. 3) behind the variator 2, which is moved in the predetermined relationship to the variator 2, the graph shown in FIG. 10 changes to the one shown in FIG. 12. For example, curve 13 represents 50 cm, curve 14 represents 1 meter, curve 15 represents 2 meters, curve 16 represents 10 meters and curve 17 represents infinite.
In the inner focus lens or the rear focus lens, it can focus to a very near object compared to the front focus lens and a compact lens structure is attained depending on a type of lens. On the other hand, in order to prevent defocusing in zooming even if the distance to the object does not change, the relationship as shown in FIGS. 13 or 12 should be exactly maintained.
The following methods have been proposed to avoid such defocusing.
First, a combination with a TTL auto-focusing device is pointed out. For example, in a video camera auto-focusing device, a peak of a high frequency component of an image pickup signal of a CCD is used to indicate an in-focus position.
FIG. 13 shows a principle thereof. An abscissa represents a position of a focusing lens group and an ordinate represents a high frequency component (focusing voltage) of an image pickup signal. The focusing voltage is maximum at a position shown by an arrow, and the position A is an in-focus position of the lens. A front focus or a rear focus in the defocus status is determined by whether it is on the left side or the right side of the point A.
Several methods to determine the front focus and the rear focus have been known.
In a first method, the focusing lens group is moved to one of the two directions and a change in the focusing voltage caused thereby is detected.
In a second method, an image pickup device is slightly vibrated by a piezo-electric actuator to detect a phase relationship of a change in the focusing voltage.
In a third method, the focusing lens group itself is slightly vibrated in a similar manner to that of the second method. It is usually carried out in the inner focus lens by using a stepping motor.
In the inner focus lens, when the lens is zoomed from the in-focus position at the wide end to the telescope end, the lens may follow a distance locus in the positional relation between the focal length and the focusing lens group as shown in FIGS. 10 and 12. However, when a focus depth is taken into consideration, it is not possible to exactly predict from an absolute position of the focusing lens group which is at the in-focus position in the wide end to the in-focus position in the telescope end.
Accordingly, when the lens is zoomed from the wide end to the telescope end, it is essential to always operate the auto-focusing device to maintain the in-focus state based on the detection by the auto-focusing device.
When the first to third methods are to be applied to the zooming of the inner focus lens from the wide end to the telescope end, the second method which uses the actuator such as a piezo-electric actuator separately from the actuator for driving the focusing lens group and vibrates the image pickup device other than the focusing lens group is not disadvantageous to the inner focus lens and the direction can be detected, but it is inadvantageous in terms of cost and size.
On the other hand, the first and third methods need the actuator to drive the focusing lens group in order to follow the locus shown in FIGS. 10 or 12. It is necessary to superimpose the drive for detecting the direction on the following operation.
In this case, there is no problem in terms of cost and size but a specification required for the actuator is severe. Accordingly, when it is spaced from the in-focus point in the map shown in FIGS. 10 or 12, the "follow" information is no longer the information of locus which passes through the in-focus point. As a result, the ability to detect the direction is lowered.